Since melt-blown nonwoven fabrics can be composed of ultrafine fibers as compared with spun bonded nonwoven fabrics, they have excellent flexibility and are used not only for filters but also for sanitary materials, clothes, packaging materials, battery separators, etc. by using them singly or laminating them onto others such as nonwoven fabrics.
For producing the melt-blown nonwoven fabrics, a molten resin is subjected to drawing and thinning by means of a fluid of high temperature and high velocity, and therefore, ball-like substances (shots) and fly-like substances are liable to be formed, and various methods to solve them have been proposed.
For example, there have been proposed various methods, such as a method in which a shape of a die nose tip of a melt-blowing die, the distance between a die nose tip and a lip plate tip, etc. are set in the specific ranges (patent literature 1: Japanese Patent Laid-Open Publication No. 1979-103466), a method in which a die having a lip plate tip width (interval between air knives) of 0.4 to 0.8 mm is used (patent literature 2: Japanese Patent Laid-Open Publication No. 1992-91267), a method in which a melt-blown nonwoven fabric is produced under the conditions of a nozzle orifice diameter of 0.1 to 0.5 mm and a discharge rate per single hole of 0.05 to 0.8 g/min, preferably 0.1 to 0.5 g/min (patent literature 3: Japanese Patent Laid-Open Publication No. 1993-295645), a method in which the spacing (air gap) of a drawing fluid flow path, the distance between a die nose tip and a lip plate tip, and the ratio between them are set in the specific ranges (patent literature 4: Japanese Patent Laid-Open Publication No. 2009-200135) and a method in which the mean fiber diameter is set in the range of 0.1 to 5.0 μm (patent literature 5: Japanese Patent Laid-Open Publication No. 1992-163353).
As a method to obtain a melt-blown nonwoven fabric of thin fibers, a method in which secondary blowing air having a temperature of not lower than 50° C. is blown against spun filaments transversely to delay cooling and solidification of the spun filaments, whereby thinning is carried out (patent literature 6: Japanese Patent Laid-Open Publication No. 2006-83511) has been proposed.
In the existing circumstances, melt-blown nonwoven fabrics having a little narrower fiber diameter distribution can be produced by the above methods proposed, but formation of thick fibers each having a fiber diameter of twice or more the mean fiber diameter caused by fusion bonding of fibers during the melt extrusion cannot be completely prevented.
In the method in which the mean fiber diameter is set in the range of 0.1 to 5.0 μm (patent literature 5: Japanese Patent Laid-Open Publication No. 1992-163353), a melt-blown nonwoven fabric wherein the coefficient of variation (CV) of fiber diameter was not more than 30% was obtained, but when the melt-blown nonwoven fabric obtained by the method described in the patent literature 5 was evaluated by the evaluation method described in the examples of the present invention, the coefficient of variation (CV) of fiber diameter was 90%.